1. Field of the Invention
Past research has suggested that only a small fraction of our nation's energy demand can be met with fuel produced from solid wastes. However, research presently being conducted suggests that hydrogen produced from solid wastes could economically meet the nation's entire natural gas demand. Alternatively, large quantities of methanol could be produced by this system for use as a motor fuel.
The system of this invention uses solar energy to provide heat for the pyrolysis of solid wastes and the gasification of the remaining char. Pyrolysis of solid wastes results in the evolution of CO.sub.2, CO, H.sub.2, CH.sub.4 and various other gases, tars, oils, and char. The gaseous and liquid by products are catalytically converted to H.sub.2, CO, CH.sub.4, and CO.sub.2 in a steam atmosphere using a commercial nickel catalyst. The remaining char, CO, CO.sub.2, and CH.sub.4 are catalytically reacted according to the following formulae: EQU CH.sub.4 + 2H.sub.2 O .fwdarw. CO.sub.2 + 4H.sub.2 (3) EQU co.sub.2 + c .fwdarw. 2co (4) EQU co + h.sub.2 o .fwdarw. co.sub.2 + h.sub.2 ( 5) EQU c + h.sub.2 o .fwdarw. co + h.sub.2 ( 6)
thus pyrolysis of solid wastes in a steam atmosphere has been used to manufacture a producer gas containing H.sub.2 CO, and CO.sub.2. The CO can be shifted to hydrogen, or reacted with the hydrogen already present to produce methanol using the commercial reaction EQU CO + 2H.sub.2 .fwdarw. CH.sub.3 OH . (2)
alternatively, if a producer gas rich in CO is desired, some CO.sub. 2 can be added to the steam reactant to produce excess CO via reaction (4).
Reactions (4) and (6) are normally observed at temperatures above 800.degree. C and such temperatures impose severe engineering problems. Catalysts have been used to lower the temperature range for the practice of reaction (6); however, catalysts have not been discovered for reaction (4) using pyrolytic char from solid wastes as a source of carbon. The catalysts cobalt molybdate, NaHCO.sub.3, and other alkali metal catalysts have been successfully employed to facilitate reaction (4). The catalysts were dissolved in water and deposited on the char by soaking the char in the catalyst-water solution and subsequently vaporizing the water. Since reactions (3), (5), and (6) require water (steam) this method of catalyst deposition is well suited to the system of interest. The catalyst is recovered by soaking the ash residue remaining after the gasification of the solid wastes in water. Tables 1 and 2 illustrate the effect of the catalysts on reaction (4) for representative space velocities and temperatures.
Table I ______________________________________ BLANK RUNS ______________________________________ Description: CO.sub.2 was reacted with char produced by the Monsanto Process containing no catalyst Temperature Space Velocity % Conversion ______________________________________ 700.degree. C 59.3 cm.sup. 3 /min trace 750.degree. C 59.3 cm.sup.3 /min 4% 750.degree. C 10.8 cm.sup.3 /min 9% ______________________________________
TABLE II ______________________________________ CATALYST RUNS ______________________________________ Description: CO.sub.2 was reacted with char produced by the Monsanto Process. The cata- lyst was deposited on the char by the method described in the text Temp. Space Velocity Catalyst % Conver. ______________________________________ 700.degree. C 59.3 cm.sup.3 /min NaHCO.sub.3 5% 750.degree. C 59.3 cm.sup.3 /min NaHCO.sub.3 12% 750.degree. C 59.3 cm.sup.3 /min Cobalt 8% Molybdate 750.degree. C 10.8 cm.sup.3 /min Cobalt 16% Molybdate ______________________________________
2. Prior Art
The PUROX System (covered by U.S. Pat. No. 3,729,298) was developed by Union Carbide Corporation in response to the need for advanced solutions to the problems of solid waste disposal and resource recovery. The PUROX System utilizes oxygen, instead of air, to produce high-temperature incineration and pyrolysis of all types of refuse. The only products formed are a compact, sterile residue and a fuel gas valuable as a cleanburning source of energy. The basic PUROX System consists of a vertical shaft furnace into which refuse is fed through a charging lock at the top. Oxygen is injected into the combustion zone at the bottom of the furnace where it reacts with carbon char residue from the pyrolysis zone. The temperature generated in the hearth is sufficiently high to melt and fuse all noncombustible materials. The molten material continuously overflows from the hearth into a water quench tank where it forms a hard, sterile granular product. The hot gases formed by the reaction of oxygen and carbon char rise through the descending waste. In the middle portion of the vertical shaft furnace, organic materials are pyrolyzed under an essentially reducing atmosphere to yield a gaseous mixture high in carbon monoxide and hydrogen (typically about 50% CO and 30% H.sub.2 by volume on a dry basis). As the hot gaseous products continue to flow upward, they dry the entering refuse in the upper zone of the furnace. The high thermal efficiency of PUROX System is indicated by the relatively low temperature (about 200.degree. F) of the by-product gas exiting through a duct to the gas cleaning section of the system. As it leaves the furnace, the gas mixture contains water vapor, some oil mist formed by the condensation of high-boiling organics, and minor amounts of fly ash. The oil mist and fly ash solids are removed by a gas cleaning system. After cleaning, the product gas is passed through a condenser. The resultant dry gas is a cleanburning fuel, comparable to natural gas in combusion characteristics. Its heating value is approximately 300 BTU/cu.ft. This recovered gas can be used effectively as a supplementary fuel in an existing utility boiler or other fuel-consuming operations without downrating of the boiler or making extensive and costly boiler modifications. Because the gas produced by the PUROX System is essentially sulfur-free and contains only about one-tenth the amount of fly ash allowable under federal air quality standards, it is an ideal fuel for all types of existing gas-fired furnaces.
The system produces four times as much energy as it consumes. Only 20% of the total energy recovered by the system is needed to meet all of its internal energy requirements, including that consumed to produce oxygen used in the furnace. The remaining 80% is available for other fuel applications. This is an important recovered resource, particularly in view of the growing shortage of clean fuels. The granular solid residue produced from the noncombustible portions of the refuse is free of any biologically active material. The volume of solid by product is only about 2 to 3 percent of the volume of incoming refuse, depending upon the amount of noncombustible materials in the mixed wastes. By contrast, a well-designed and efficiently-operated conventional incinerator produces a solid residue volume of 10% or more of the volume of refuse burned. Importantly, the dense granular residue produced by the PUROX System is considered suitable as a construction fill material or for other potentially valuable uses. The PUROX System is notable in another respect. It is designed to use only a small fraction of the oxidant gas required in conventional incineration. The PUROX System requires only one-fifth of a ton of oxygen per ton of refuse, while a conventional incinerator requires approximately seven tons of air per ton of solid waste burned. This 35-fold difference in oxidant gas flow means that the PUROX System will produce only one-twentieth as much gas volume to be cleaned. This factor, in turn, makes it possible to reduce fly ash content in the gaseous emissions to less than one-tenth of that attainable with a conventional incinerator. Combustion of the fuel gas from the PUROX System produces emissions far below the allowable maximum specified by federal air quality standards. The use of oxygen enables the PUROX System to process effectively solid waste of widely varying composition. This flexibility is especially advantageous in adapting to operating variations which commonly result from seasonal, regional, and socio-economic factors. Another important feature of this System is its compatibility with other solid waste disposal facilities either new or existing. It can readily handle refuse in "as received" condition, or it can be used to treat refuse which has been preprocessed by shredding, separation, or resource recovery operations in existing equipment.